An integrating sphere is used for measuring the intensity of light to be measured when the light to be measured is generated radially. A high-diffuse reflecting powder is coated on a spherical main body inner wall of the integrating sphere, and when the light to be measured is generated radially, it is multiply diffused and reflected by the high-diffusion reflecting powder. This diffuse-reflected light enters a photodetector, and an output signal of the photodetector is led to a light intensity meter, and thus the intensity of the emitted light to be measured can be measured. Such an integrating sphere is disclosed in Patent Documents 1 to 3 listed below.    Patent Document 1: Japanese Patent Publication No. 2517102    Patent Document 2: Japanese Patent Publication No. 2811565    Patent Document 3: Japanese Patent Publication No. S54-35114